1. Near-Earth Magnetotail Reconnection and Plasmoid Formation in Connection With a Substorm Onset on 27 August 2001 S. Eriksson 1, M. Oieroset 2, D. N. Baker 1, C. Mouikis 3, M. W. Dunlop, H. Reme, R. E. Ergun 1, and A. Balogh M. W. Dunlop 4, H. Reme 5, R. E. Ergun 1, and A. Balogh 6 1 Laboratory for Atmospheric and Space Physics, University of Colorado at Boulder, USA 2 Space Sciences Laboratory, University of California at Berkeley, USA 3 Space Science Center, University of New Hampshire, USA Rutherford Appleton Laboratory, UK 4 Rutherford Appleton Laboratory, UK Centre d’Etude Spatiale des Rayonnements, France 5 Centre d’Etude Spatiale des Rayonnements, France Imperial College, UK 6 Imperial College, UK Contact information: eriksson@lasp.colorado.edu

2. Outline Introduction Introduction 1. Tail reconnection and slow-mode shocks 2. deHoffmann-Teller and Walen analyses Cluster observations and Walen analyses Cluster observations and Walen analyses August 27 2001 from 03:50 UT to 04:35 UT (post-midnight event) Summary Summary

3. Tail Reconnection and Slow-mode Shocks [Hill, T.W., JGR, 80, 4689, 1975][Feldman et al., JGR, 92, 83, 1987] Acceleration in tail reconnection is assumed to take place across a slow- mode shock connected to the diffusion region. Configuration essentially that of Petschek [1964] (above right).

4. Tail Reconnection and Slow-mode Shocks [Hill, T.W., JGR, 80, 4689, 1975][Feldman et al., JGR, 92, 83, 1987] Slow-mode shocks are defined in ideal MHD by an increased plasma pressure and a decreased B-field strength where the B-field bends toward the shock normal.

5. Tail Reconnection and Slow-mode Shocks [Hill, T.W., JGR, 80, 4689, 1975][Feldman et al., JGR, 92, 83, 1987] Slow-mode shocks were first observed in the tail by e.g. Feldman et al. [1984, 1987] using the Rankine-Hugoniot jump conditions. Also suggested by Oieroset et al. [2000] who successfully used the Walen analysis on Wind data 60 Re downtail.

6. deHoffmann-Teller Analysis [Khrabrov and Sonnerup, ISSI Sci.rep., 1998] The existence of an HT frame indicates the presence of a quasi- stationary coherent pattern of magnetic field and plasma velocity. The HT interval should include data points on either side of the boundary.

7. deHoffmann-Teller Analysis [Khrabrov and Sonnerup, ISSI Sci.rep., 1998] A Galilean HT frame transformation allows for the identification and analysis of such coherent structures (e.g. slow-mode shocks) moving past an observing platform.

8. Walen Analysis as a Test of Reconnection (Not a slow shock since B does not bend toward normal) Reconnection predicts that the generated “rotational discontinuity” (finite normal B-field component) propagates away from the diffusion region at a field-aligned phase speed in the HT shock frame and its magnitude is determined by the type and strength of the shock.

9. Walen Analysis as a Test of Reconnection (Not a slow shock since B does not bend toward normal) The sign of the Walen slope depends on whether the B-field is parallel or antiparallel to the flow direction.

10. Walen Analysis in the Magnetotail Walen slope Oieroset et al., JGR, 105, 25,247, 2000. Region I: earthward jet, Bx>0, R W <0 Region II: tailward jet, Bx>0, R W >0

11. Walen Analyses at Wind (x= -60 Re) Oieroset et al., JGR, 105, 25,247, 2000. Note that there is no ion composition in the Wind data set and that these slopes were derived assuming 100% H +. Earthward jet interval Negative Bx Walen slope should be positive

12. Cluster Location August 27 2001

13. Cluster Observations on August 27 2001 Density (CODIF) Plasma  (CODIF) Ion speed (HIA) [Vx, Vy, Vz] Alfven Mach number Note: M A <1 Magnetic field magnitude ABC [Bx, By, Bz]

14. Cluster Observations on August 27 2001 Density (CODIF) Plasma  (CODIF) Ion speed (HIA) [Vx, Vy, Vz] Alfven Mach number Note: M A <1 Magnetic field magnitude [Bx, By, Bz] Approximate substorm onset in global UV images at ~04:06-04:08 UT [Baker et al., GRL, 2002] ABC

15. Comparison with the Substorm of April 24 1979 Overview of ISEE 2 magnetic field (GSE) and plasma data near substorm onset. [Hones et al.,1986; Feldman et al., 1987] Bx By Bz beta Vx sc1 sc3 Bx By Vx PpPp Bz

16. Comparison with the Plasmoid of April 24 1979 Three-dimensional magnetic field structure of plasmoid at substorm onset [from Hones et al., GRL, 1982]. Cluster magnetic field hodogram (GSM)

17. Walen Analyses on August 27 2001 (A) Tailward Region II Jets 04:00-04:03 UT sc1 sc3 Positive Bx Tailward jets Prediction: Positive Region II Walen slope

18. Walen Analyses on August 27 2001 (B) Earthward Region I Jets 04:06-04:10 UT sc1 04:06-04:08 UT Positive Bx Earthward jets Prediction: Negative Region I Walen slope sc3 04:09-04:10 UT

19. Walen Analyses on August 27 2001 (C) Earthward Region I Jets 04:26-04:28 UT sc1 sc3 Positive Bx Earthward jets Prediction: Negative Region I Walen slope

20. x: black y: green z: red Angle between V and B …. BAC

21. Substorm of 27 August 2001 (A)

22. Substorm of 27 August 2001 (B)

23. Substorm of 27 August 2001 (B) (C)

24. Note on Importance of Density for the Walen Slope A mix of 5% O + ions and 95% H + results in an effective mass m=1.75m p and a lower Alfven speed, that effectively should increase the Walen slope ….

25. Impact of m=mp Assumption O+ fraction: 0.8% - 4.8% Avg O+ fraction: 1.8% Walen slope=0.73 Assume 100% H+ Walen slope=0.65

27. Summary The Walen test together with the location of Cluster north of the neutral sheet confirms the presence of a slow mode shock on the tailward side of the near-Earth X-line on August 27, 2001 that most likely accelerated the ions. The Walen test together with the location of Cluster north of the neutral sheet confirms the presence of a slow mode shock on the tailward side of the near-Earth X-line on August 27, 2001 that most likely accelerated the ions. The Walen test for some earthward jets may indicate a correct slope (sign), although the quality of the HT frame is lower. The Walen test for some earthward jets may indicate a correct slope (sign), although the quality of the HT frame is lower. The earthward jets seem to be more field-aligned which implies that ideal MHD (and thus the HT analysis) breaks down. The earthward jets seem to be more field-aligned which implies that ideal MHD (and thus the HT analysis) breaks down. The use of ion composition data from CODIF improved the Walen slope. The use of ion composition data from CODIF improved the Walen slope.